His story would have ended there, but his body was one of the many abducted by the Cybermen to perform experiments on in order to create living Cybermen out of corpses to aid in their wars. Ironically, he was subjected to similar experiments that the Nazis themselves carried out-albeit on living test subjects.
Hermann was upgraded into a Cyberman and transported by the Cybermen to Mondas. 40 years later, in the 1980s, Hermann's Cyberform returned to Earth on the expedition by the Cybermen to conquer Earth. Hermann was destroyed completely during the invasion after his power source cable was cut by a British soldier, and his Cyberform crumbled away and died.
Hermann is one of the longest lived Cybermen to be created from World War II test subjects. Many of them died soon after their conversions, and what few survived were killed in the succeeding Cyber Wars.